Kinetic, gravitational, elastic, thermal, chemical, nuclear and electromagnetic energy stores — and the pathways that move energy between them.
⚡ Name and describe all eight energy stores used in AQA GCSE Physics
🔢 Use the kinetic energy equation: Ek = ½mv²
🏔️ Use the gravitational potential energy equation: Ep = mgh
🌀 Use the elastic potential energy equation: Ee = ½ke²
🔄 Identify the four energy transfer pathways: mechanical, electrical, heating, radiation
📊 Describe energy transfers in real-world systems and apply conservation of energy
What is an Energy Store?
Energy is a property of an object or system — it describes the capacity to cause change. In AQA GCSE Physics we think about energy being held in stores and moved between them via transfer pathways. Energy cannot be created or destroyed — it can only be transferred from one store to another. This is the Law of Conservation of Energy.
Energy Store: A way of describing where energy is "held" within an object or system at a particular moment.
AQA specifies eight energy stores you need to know:
Kinetic
Gravitational potential
Elastic potential
Thermal (internal)
Chemical
Nuclear
Electrostatic
Magnetic
💡 The total energy in a closed system always stays the same. If one store decreases, another must increase by the same amount.
When we say energy is "used up" in everyday language, we really mean it has been transferred to stores (often thermal stores in the surroundings) that are no longer useful to us. This is called dissipation.
Kinetic & Gravitational Potential Energy Stores
These two stores are the most commonly calculated at GCSE and are closely linked — objects often convert between them (e.g. a roller coaster).
Kinetic Energy (Ek)
Any object that is moving has energy in its kinetic store. It depends on the object's mass and its speed.
Ek = ½ × m × v²
Symbol
Quantity
SI Unit
Ek
Kinetic energy
Joules (J)
m
Mass
Kilograms (kg)
v
Speed
Metres per second (m/s)
Notice that speed is squared — doubling the speed quadruples the kinetic energy. This is why speed is so dangerous for vehicles.
Gravitational Potential Energy (Ep)
An object raised above a reference point has energy stored in its gravitational potential store due to its position in a gravitational field.
Ep = m × g × h
Symbol
Quantity
SI Unit
Ep
Gravitational potential energy
Joules (J)
m
Mass
Kilograms (kg)
g
Gravitational field strength
N/kg (use 9.8 or 10 as directed)
h
Height above reference point
Metres (m)
💡 On Earth, g = 9.8 N/kg. Some AQA questions allow g = 10 N/kg — always check the question.
Elastic Potential Energy Store
When a spring, elastic band, or any elastic material is stretched or compressed, energy is stored in its elastic potential store. When released, this energy is transferred — usually to the kinetic store of a projectile or the spring itself.
Ee = ½ × k × e²
Symbol
Quantity
SI Unit
Ee
Elastic potential energy
Joules (J)
k
Spring constant
Newtons per metre (N/m)
e
Extension (or compression)
Metres (m)
Spring constant (k): A measure of a spring's stiffness. A larger k means a stiffer spring — more force is needed for the same extension.
This equation only applies when the spring is within its limit of proportionality (i.e. it obeys Hooke's Law). Beyond this point the spring is permanently deformed and the equation is no longer valid.
💡 Like kinetic energy, the extension is squared — doubling the stretch quadruples the stored elastic potential energy.
Other Energy Stores
Thermal (Internal) Energy Store
All matter is made of particles that are constantly moving and vibrating. The thermal energy store relates to the total kinetic and potential energy of all the particles in an object. The hotter an object, the greater its thermal energy store. This store increases when an object is heated.
Chemical Energy Store
Energy stored in the bonds between atoms and molecules. This store decreases when fuels combust, batteries discharge, or food is metabolised. Examples include petrol, food, batteries, and explosives. Chemical reactions break and form bonds — energy is released when the new bonds formed are stronger than those broken.
Nuclear Energy Store
Energy stored in the nucleus of atoms. This is released during nuclear fission (splitting large nuclei, e.g. uranium-235) or nuclear fusion (joining small nuclei, e.g. hydrogen). Nuclear power stations use fission; the Sun uses fusion. The energy released is enormous compared to chemical reactions.
Electrostatic Energy Store
Energy stored when charged objects exert forces on each other — for example, two charged parallel plates, or static charge on a balloon after rubbing. Lightning is a dramatic release from an electrostatic store.
Magnetic Energy Store
Energy stored in magnetic fields, for example between two magnets held apart. When magnets attract and snap together, energy is transferred from the magnetic store to kinetic and thermal stores.
Energy Transfer Pathways
Energy moves between stores via transfer pathways. AQA GCSE identifies four main pathways:
1. Mechanically: A force doing work on an object. E.g. pushing a box transfers energy from the chemical store in your muscles to the kinetic store of the box. Work done = force × distance.
2. Electrically: Charge flowing around a circuit, carrying energy from one component to another. E.g. a battery (chemical store) → light bulb (thermal and light stores) via an electric current.
3. By Heating: Energy transferred from a hotter region to a cooler region via conduction, convection, or radiation (infrared). E.g. a hot pan heats water by conduction.
4. By Radiation: Energy carried by electromagnetic waves (including light, infrared, radio waves, etc.). E.g. the Sun's nuclear store → Earth's thermal store via radiation.
A useful way to describe a system is with an energy transfer diagram or a Sankey diagram. These show the stores at the start and end, connected by labelled arrows showing the transfer pathways.
💡 Example: A falling ball — GPE store → Kinetic store (mechanical pathway). On impact — Kinetic store → Thermal store + Sound (mechanical pathway causing deformation and vibration).
In all real processes, some energy is dissipated — spread out into the thermal store of the surroundings where it is no longer useful. This is why no machine is 100% efficient.
Conservation: Total energy at start = Total energy at end (in a closed system)
A car of mass 1200 kg is travelling at 30 m/s. Calculate its kinetic energy.
1 Write down the equation: Ek = ½ × m × v²
2 Identify the values: m = 1200 kg, v = 30 m/s
3 Square the speed: v² = 30² = 900 m²/s²
4 Substitute: Ek = ½ × 1200 × 900
5 Calculate: Ek = 600 × 900 = 540 000 J
Ek = 540 000 J = 540 kJ
A box of mass 5 kg is lifted onto a shelf 2.4 m above the floor. Calculate the increase in gravitational potential energy. (g = 9.8 N/kg)
1 Write down the equation: Ep = m × g × h
2 Identify the values: m = 5 kg, g = 9.8 N/kg, h = 2.4 m
3 Substitute: Ep = 5 × 9.8 × 2.4
4 Calculate: Ep = 5 × 23.52 = 117.6 J
Ep = 117.6 J
A spring has a spring constant of 400 N/m and is compressed by 0.05 m. Calculate the elastic potential energy stored in the spring.
1 Write down the equation: Ee = ½ × k × e²
2 Identify the values: k = 400 N/m, e = 0.05 m
3 Square the extension: e² = 0.05² = 0.0025 m²
4 Substitute: Ee = ½ × 400 × 0.0025
5 Calculate: Ee = 200 × 0.0025 = 0.5 J
Ee = 0.5 J
A ball of mass 0.2 kg is dropped from a height of 10 m. Assuming all GPE converts to kinetic energy, calculate the speed of the ball just before it hits the ground. (g = 9.8 N/kg)
1 Calculate the GPE at the top: Ep = m × g × h = 0.2 × 9.8 × 10 = 19.6 J
2 By conservation of energy, all GPE becomes KE: Ek = 19.6 J
3 Use Ek = ½mv² and rearrange for v: v² = (2 × Ek) ÷ m
Question 1: Which of the following is NOT one of the eight AQA energy stores?
Question 2: A skateboarder of mass 60 kg is moving at 4 m/s. Which answer correctly calculates the kinetic energy?
Question 3: Which energy transfer pathway is used when a kettle heats water through its element?
Question 4: A 2 kg book is placed on a shelf 1.5 m high. Calculate the gravitational potential energy stored. (g = 10 N/kg) — type your answer in J.
Question 5: A spring with k = 200 N/m is stretched by 0.1 m. Calculate the elastic potential energy stored in the spring — type your answer in J.
Challenge 1 (Higher): A rollercoaster car of mass 800 kg starts from rest at the top of a 40 m drop. Assuming no energy is lost to friction, calculate the speed of the car at the bottom of the drop. (g = 9.8 N/kg)
✅ Step 1: Calculate GPE at the top: Ep = m × g × h = 800 × 9.8 × 40 = 313 600 J Step 2: All GPE converts to KE (no friction): Ek = 313 600 J Step 3: Rearrange Ek = ½mv² → v² = 2Ek/m = (2 × 313 600) / 800 = 784 m²/s² Step 4: v = √784 = 28 m/s
Challenge 2 (Higher): A spring launcher fires a ball of mass 0.05 kg. The spring has a constant of 800 N/m and was compressed by 0.06 m. Assuming all elastic potential energy transfers to the kinetic store of the ball, calculate the ball's launch speed.
✅ Step 1: Calculate elastic PE stored: Ee = ½ × k × e² = ½ × 800 × 0.06² = ½ × 800 × 0.0036 = 1.44 J Step 2: All Ee becomes KE: Ek = 1.44 J Step 3: v² = 2Ek/m = (2 × 1.44) / 0.05 = 2.88 / 0.05 = 57.6 m²/s² Step 4: v = √57.6 ≈ 7.6 m/s
Challenge 3 (Extended Answer): A student drops a rubber ball from 2 m. It bounces back up to only 1.6 m. The ball has a mass of 0.1 kg. (g = 10 N/kg) (a) Calculate the GPE at the start. (b) Calculate the GPE after the first bounce. (c) Calculate the energy transferred to the thermal store during the bounce. (d) Explain, using ideas about energy stores and transfer pathways, why the ball does not bounce back to its original height.
✅ (a) Ep = m × g × h = 0.1 × 10 × 2 = 2 J (b) Ep = 0.1 × 10 × 1.6 = 1.6 J (c) Energy dissipated = 2 − 1.6 = 0.4 J (d) When the ball hits the ground, energy is transferred from the kinetic store to the elastic potential store of the ball (as it deforms). However, not all of this transfers back to the kinetic store — some is transferred to the thermal store of the ball and surroundings via heating (caused by the inelastic deformation). This means the ball has less kinetic energy as it leaves the ground, so it rises to a lower height. Energy is conserved overall (2 J total) but some is dissipated as heat, making the process irreversible.
Challenge 4 (Higher): A cyclist and their bicycle have a combined mass of 90 kg. They are travelling at 12 m/s along a flat road. They then cycle up a hill, gaining 15 m of height, and slow to 5 m/s at the top. Calculate the energy transferred to the thermal store due to friction during the climb. (g = 9.8 N/kg)
✅ Step 1: KE at bottom = ½ × 90 × 12² = ½ × 90 × 144 = 6480 J Step 2: KE at top = ½ × 90 × 5² = ½ × 90 × 25 = 1125 J Step 3: GPE gained = 90 × 9.8 × 15 = 13 230 J Step 4: Total energy at start = 6480 J; Total useful energy at top = KE + GPE = 1125 + 13 230 = 14 355 J Step 5: Energy transferred to thermal (friction) = Energy used − Energy at bottom → We need to account for the chemical energy input from the cyclist too. However, interpreting this as a purely mechanical problem:
Energy dissipated = KE(bottom) − KE(top) − GPE(gained) = 6480 − 1125 − 13 230 = −7875 J
The negative value confirms the cyclist must have done work (used chemical energy from food). If we assume the question asks only about the mechanical energy decrease: the cyclist's chemical store supplies additional energy. The energy lost to friction = KE lost − GPE gained = (6480−1125) − 13230 = 5355 − 13230 → Since GPE exceeds KE decrease, the cyclist pedalled. Thermal loss = KE(bottom) + Work by cyclist − KE(top) − GPE. Without work by cyclist given, we interpret: thermal energy from friction ≈ 0 J from the given stores if cyclist input is ignored, but a better interpretation: energy dissipated = (KE at bottom − KE at top) − GPE gained = 5355 − 13230 = −7875 J, confirming 7875 J came from the cyclist's chemical store. If 10% is lost to friction: this question is best answered as — Energy dissipated = (Total input energy) − (useful output) — making this an efficiency question requiring cyclist's work input. For exam purposes: dissipated energy = KEbottom − KEtop − GPEgained if all values given; here the cyclist provides additional energy so the thermal loss cannot be determined without that value. ⚠️ This question illustrates the importance of identifying all energy inputs!